When stars form they can have a large range of sizes. The sizes are determined by many factors such as the size of the nebula they were formed from, the amount of turbulence in the gas and dust, number and type of nearby stars (if any) and many other factors. Star sizes vary from small stars that are a fraction the size of the Sun to stars that are many hundreds of times larger. Directly related to the size of the star is its mass and it is the mass of a star that determines just about everything else about it.

As we saw earlier stars are powered by turning Hydrogen into Helium. This is called nuclear fusion and is the same process that takes place when a hydrogen bomb is detonated. More details here >>

A large star has a lot of mass (because there is more of the star) which means that there is more material pressing down on the centre of the star creating a higher pressure. This higher pressure forces the hydrogen atoms closer to each other increasing the probability that two Hydrogen atoms will fuse and become Helium. This results in a faster reaction creating more energy and light but because the reaction is faster it uses up the available Hydrogen more quickly.

Large stars are hotter and brighter but live relatively short lives, something in the order of a few hundred million years. Conversely, small stars are cooler and dimmer and have lifetimes of billions of years. The colour of a star is related to the temperature of the star which in turn is related to its mass which is related to its size. Large stars then are blue colour and small stars are red. The Sun is of intermediate size and mass and is a yellow star.

This relationship between the various characteristics of a star means that if you know one property you can determine the others. For instance if you see a star that is blue in colour you also know that it is large, hot, has a large mass and a relatively short life. A red star is small, cool, of low mass and long life. This relationship between these characteristics has given rise to a simple but very useful classification system.

STAR TYPES

TYPE

COLOUR

TEMPERATURE

O

B

A

F

G

K

M

Blue

Blue

Blue

Blue/White

White/Yellow

Orange/Red

Red

Over 25,000º

11,000º – 25,000º

7,500º – 11,000º

6,000º – 7,500º

5,000º – 6,000º

3,500º – 5,000º

Less than 3,500º

The Sun is a G type star. Our nearest star, the double star Alpha Centauri, consists of a G star and a K star. The brightest star in the sky, Sirius is an A type star. Procyon in Canis Minor is an F type.

A simple mnemonic to help you remember the order of the letters - O Be A Fine Girl (or Guy) Kiss Me.